The present invention relates to the generation of a plasma inside a chamber which is maintained at a low pressure, and more particularly to measurement of the electric field within the chamber.
A plasma is created and sustained by establishment of an RF electric field in a low pressure plasma region within a chamber which is supplied with a gas that can be ionized by the electric field. The electric field is introduced into the chamber by supplying RF electric energy to a plasma electrode which delimits one end of the plasma region. Such a plasma is employed in the performance of a variety of industrial and scientific processes. For example, plasmas are utilized in the semiconductor fabrication industry for the performance of deposition and etching operations on a semiconductor substrate, or wafer. A wafer to be processed is typically mounted on a wafer chuck which is disposed opposite the plasma electrode and generally delimits another end of the plasma region.
It is usually desirable to establish conditions which are as uniform as possible across the surface of the wafer so that the resulting etching or deposition procedure will have an effect which is as uniform as possible across the wafer surface. Achievement of this goal is influenced to a substantial extent by the degree of uniformity of the RF electric field within the plasma region. Therefore, a detailed knowledge of the electric field pattern within a plasma region would be of considerable value in understanding the processes being performed in the plasma, and would be of considerable assistance in the design and adjustment of plasma generating systems.
Any effort to obtain such knowledge must take into account the fact that the RF electric field in a plasma region includes substantial energy components at a number of frequencies that are harmonics of the fundamental frequency and the trend in this field is toward ever higher RF frequencies and ever higher plasma densities. The harmonic content of the RF field in the plasma increases with increasing RF fundamental frequency and increasing plasma density. As the frequency and/or the plasma density increases, so does the tendency of the electric field to become nonuniform across the surface of the plasma electrode and within the plasma region.
It has already been proposed in the prior art to map the characteristics of a volume of plasma in a plasma region by directing light emissions from different points in the plasma region to a photo sensitive detector. It has also been proposed in the prior art to monitor a plasma process with the aid of a diagnostic head assembly that can include a quartz crystal microbalance, an optical endpoint detector and a Langmuir probe.
It has further been proposed to project linearly polarized light through a plasma and to detect the light after it has passed through the plasma as a means for measuring the electron density, conductivity and/or temperature of the plasma. It is also known in the prior art to collect spectral data characterizing an emission of light from an etch species contained in a plasma during an etch process for the purpose, inter alia, of identifying the presence of foreign material faults within the plasma region.
Systems for sensing the RF current level delivered to a plasma have also been proposed, as have methods for detecting the endpoint of a plasma process by detecting emission spectra in a specific wavelength band of an active species in the plasma. According to other prior art teachings, the endpoint of a post treatment is detected by detecting discharge characteristic values at an RF electrode.
Also in the prior art, inductance probes has been employed for various purposes, such as measuring the stability and magnetic confinement of a plasma. Known probes of this type have a capability of detecting frequencies up to several hundred MHz. Simple probes for measuring the potential of a plasma at low frequencies are also known in the art.
A complete understanding of the electric field configuration in a plasma would be greatly aided by the ability to detect electric field components at frequencies of greater than 100 MHz. Currently, there are no devices available which can be safely introduced into the high temperature environment of a plasma region and can provide an indication of the intensity, or voltage gradient, of components of the electric field in such a plasma at both the fundamental frequency and significant harmonic frequencies.
The present invention is an RF electric field probe device for measuring an RF electric field intensity in a plasma. The device is composed essentially of an electric field sensing unit and an output unit. The electric field sensing unit is composed of a first electro-optical component positionable in the plasma and operable to modulate light as a function of variations of the RF electric field in the plasma at the fundamental frequency and harmonics of the RF electric field, and a first antenna unit electrically coupled to the first component for coupling the first component to the RF electric field. The output unit is coupled to the electric field sensing unit for providing an output signal containing information relating to the magnitude and frequency of the modulation which occurs in the first component.
The invention also relates to methods for mapping a plasma region with an RF electric field sensing unit having a frequency response which encompasses fundamental and significant harmonic frequencies of the RF electric field and apparatus for moving the probe device to any selected point in the plasma region.